Get rid of legacy admin widget support.

Review Request #2430 — Created March 13, 2022 and updated

Review Board
This change ports our two remaining legacy widgets (server cache and
activity graph) to inherit from `BaseAdminWidget` instead of the old
legacy `Widget` class, and then removes support for the legacy mode.

The server activity graph has additionally been ported to use the new JS
model/view architecture. As part of this, I've modernized the code a bit
but haven't spent any time redesigning the functionality or look.

Testing Done:
Loaded the admin site and interacted with the widgets.

Reviewed at